Certain vehicle control systems involve measuring pressure parameters, such as engine oil pressure, fuel pressure, transmission fluid pressure, or brake pressure. Silicon-based pressure sensors, for example, are used in a wide variety of automotive applications, including sensing manifold absolute pressure (MAP), turbo intake air pressure, and differential fuel vapor pressure. In addition, some occupant detection systems incorporate silicon-based pressure sensors.
Some silicon-based pressure sensors, known as absolute silicon-based pressure sensors, work by sensing a differential pressure across a thin silicon membrane or diaphragm within a cell body. The silicon membrane and cell body form a cavity that is hermetically sealed under a vacuum. This cavity establishes a reference pressure on one side of the membrane. The pressure sensor outputs a voltage that is proportional to the difference in pressure between the sensed pressure and the reference pressure. To provide acceptable service, the cell body and corresponding package must be able to withstand the environment.
For absolute pressure sensing applications involving hydrogen gas media, for example, in proton exchange membrane (PEM) hydrogen fuel cells, it can be difficult for the silicon membrane of the pressure sensor to maintain a differential pressure. In particular, maintaining a reference vacuum in the cavity formed by the silicon membrane and the cell body is difficult because hydrogen diffuses or permeates through the silicon. The rate at which this diffusion or permeation occurs, known as the diffusion rate or the permeation rate, can be significant over long periods of time at temperatures commonly found in automotive environments. As a result, the reference vacuum can fill with hydrogen over time. When the reference vacuum fills with hydrogen, the accuracy of the absolute pressure sensor can deviate from acceptable limits.